Interior Secretary Ken Salazar announced this week that entrance to all of America’s national parks and monuments will be free for three weekends this summer: June 20-21, July 18-19 and August 15-16. Entrance fees, ranging from $3 to $25, are charged at 147 of the 391 parks and monuments.

The move is designed to encourage tourism, which has been declining for much of the past two decades and was likely to be adversely affected this summer by the country’s economic problems.

“During these tough economic times, our national parks provide opportunities for affordable vacations for families,” said Salazar. “I encourage everyone to visit one of our nation’s crown jewels this summer.”

Though the weekends will cost the Park Service an estimated half-million dollars a day of lost revenue, according to The Associated Press, they are expected to be a boon for local economies. Last year, according to the Department of the Interior, “Spending by non-local visitors provided $10.6 billion for local economies, supporting more than 213,000 jobs, not counting National Park Service jobs.”

The findingDulcinea Web Guide to National Parks has online resources on the history of national parks, camping and activities in national parks, parks accommodations and organizations, volunteering opportunities, international parks and national parks jobs.

A 2006 study by the U.S. Journal of Environmental Management found that after 50 years of steady increase, attendance to national parks began declining in 1988. Between 1988 and 2003, per capita visits to national parks declined 25 percent.

There are several theories for the decline, including a rise in gas prices, increased use of the Internet and electronic media, and cleaner, safer cities. The Parks Service has been introducing new measures to draw people to parks and increase children’s interest in nature, and there was actually an increase in attendance in 2006.

Some in Congress are also trying to help children embrace the outdoors, with the proposed No Child Left Inside Act. The 2009 version of the bill calls for the Department of Education to spend $100 million a year for the next five years on nature education. It is almost certainly going to be passed through the Democrat-controlled House and Senate, reports the Colorado Statesman.

The Parks Service is also hoping to increase its budget for preservation and restoration projects through government funding and private donations. The federal government created the Centennial Initiative, a program aiming to enhance national parks and monuments in the years leading up to the 100-year anniversary of the Parks Service in 2016.